Air Cooled Chillers
Air cooled chillers are refrigeration systems that cool fluids and work in tandem with the air handler system of a facility. Air cooled chillers are types of chillers that rely on the use of fans to reject heat outside the...
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This article will take an in-depth look at laser coolers and laser cooling.
The article will bring more detail on topics such as:
This chapter will discuss what laser cooling is, methods of cooling, and how laser coolers function.
Laser cooling is a multi-process that includes a number of techniques in which atomic and molecular samples are cooled down to a temperature near absolute zero. These techniques depend on the fact that when an object, which is usually an atom, re-emits a photon - a particle of light - its momentum is subject to change.
For a combination of particles, their thermodynamic temperature is proportional to the variance in their particle velocity. This means that more homogeneous velocities among particles correspond to a lower temperature.
These techniques used in the process of laser cooling combine atomic spectroscopy with the aforementioned mechanical effect of light so as to compress the distribution of the velocity of an ensemble of particles, thus leading to the cooling of the particles.
Laser cooling uses a wide range of methods. Some of these methods include Doppler cooling, which is now viewed as laser cooling itself because it is the most commonly used method.
Other methods include Sisyphus cooling, resolved sideband cooling, Raman sideband cooling, velocity selective coherent population trapping (VSCPT), gray molasses, cavity mediated cooling, polarization gradient cooling, anti-stokes cooling in solids, electromagnetically induced transparency (EIT) cooling, and the use of the Zeeman slower.
This is a mechanism that can be used to trap and slow the motion of atoms in order to cool a substance. In simple terms, a stationary atom sees the laser neither red nor blue shifted and it does not absorb the photon.
An atom that is moving away from the laser sees it as red shifted; therefore, it does not absorb the photon. However, an atom moving toward the laser sees it as blue shifted and absorbs the photon, thus slowing down the atom. This photon makes the atom excited, thus moving an electron to a higher quantum state. The atom re-emits a photon and because of the fact that its direction is random, there is no net change in momentum over many photons.
The essence of the working principle is that the vast majority of photons that come anywhere near a particular atom are almost completely unaffected by the atom. This atom is also nearly transparent to most frequencies or colors of photons. A small number of photons happens to resonate with the atom in a very few narrow bands of frequencies, which is a single color rather than a mixture of colors like white light.
The atom likely absorbs the photon for a brief moment of time when one of these photons comes closer to the atom. Then the atom re-emits a similar photon in some random and unpredictable random direction. The common idea that lasers increase the thermal energy of matter is not major in examining these individual atoms.
If an atom is practically motionless in the sense that it is a cold atom and the frequency of the laser which is focused upon it can be controlled, most frequencies do not affect the atom as it is invisible to those frequencies. There are therefore only a few points of electromagnetic frequency that have an effect on that atom. When the atom is at those frequencies, it can absorb a photon from the laser. This causes it to transition to an excited electronic state as its electron moves to a higher quantum state while also later picking up the momentum of that photon. The atom must now be expected to drift in the same direction as the photon was traveling because it now has attained the photon’s momentum.
After a short moment, the atom will now be seen emitting a photon in a random direction, which is caused as it relaxes to a lower electronic state. The atom will likely give up its attained momentum and become motionless again. This is especially likely if the photon that the atom emits is in the direction of the original photon. But if the photon is emitted in an opposite direction, the atom will therefore have to provide momentum in that opposite direction. This means that this atom will have to gain even more momentum in the direction of the original photon.
This is to be done as so in order to conserve momentum, with double its original velocity. In most cases, the photon usually speeds away in some other direction, giving the atom some sideways thrust. The frequency can be changed by changing the positioning of the laser by using a monochromatic laser light that contains a frequency a little below one of the resonant frequencies of this atom. This is the state at which the frequency will not directly affect the atom’s state.
If the laser was to be positioned so that it was moving towards the observed atoms, then the Doppler Effect would raise its frequency and at some specific velocity, the frequency would be precisely correct for the atoms contained to begin the absorbing of the photons. Beginning with a laser frequency that is well below the resonant frequency, the photons from any laser pass through the majority of atoms.
Atoms that will be moving rapidly towards a certain laser catch the photons of that laser, thus slowing down the atoms until those atoms become transparent again. That is, atoms that will be rapidly moving away from that laser are transparent to that laser’s photons, but they are rapidly moving towards the laser but directly opposite it. On a graph of the velocities of the atoms, atoms moving rapidly to the right correspond with stationary dots far to the right, and atoms moving rapidly to the left correspond with stationary dots which are far to the left.
There is a narrow band on the left side which is linked with the speed at which those same atoms start absorbing the protons from the left laser. Atoms in that sideband are the ones that interact with the left laser. When a photon from the left laser bumps into one of these atoms, it suddenly slows down an amount corresponding to the momentum of the photon. Whereas if the photon is released by the area some distance directly to the right, then the dot is redrawn that same distance to the left, also putting it back to the narrow band of interaction. But in most cases, the photon is released by the atom in some random direction and the dot is redrawn at that same quantum distance in the opposite direction. As the laser frequency increases, there is a contraction in the boundary, thus pushing all the dots on that graph towards zero velocity. This is the given definition of ‘cold’.
Sisyphus cooling is a method of laser cooling of atoms that is used to reach temperatures below the Doppler cooling limit. This type of cooling can be achieved by shining two counter-propagating laser beams with orthogonal polarization onto a sample of an atom. Atoms that are moving through the potential landscape along the direction of the standing wave result in a loss of kinetic energy. They move to maximum potential and at this point, optical pumping moves them back to a lower energy state, and this results in the lowering of the total energy of the atom.
The principle of Sisyphus cooling is based on the principle that the counter propagation of two orthogonally polarized lasers generates a standing wave which is in polarization with a gradient that is between left hand circularly polarized light and right hand circularly polarized light along the standing wave. The gradient noted occurs over a length scale of ⅄2, and then it repeats whilst it is mirrored about the y-z plane. In some positions the counter propagating beams have a phase difference of pi2. The polarization is specifically circular in nature and in cases where there is no phase difference, the polarization is linear. In the intermediate regions, there is a gradient ellipticity of the superimposed fields. In order to have a cooling effect, there must be some loss or dissipation of energy.
This method is a laser cooling technique which allows for the cooling of tightly bound atoms and ions beyond the Doppler cooling limit, potentially to the atoms’ and ions’ ground state. In general it is issued to cool strongly trapped atoms to the ground state of their motion.
An atom that is cold and trapped can be treated to a good approximation as a quantum mechanical harmonic oscillator. If the noted spontaneous rate in decay is much smaller than the vibrational frequency of the atom contained in the trap, then the energy levels of the system can be resolved as consisting of internal levels, with each corresponding to a ladder of vibrational states.
For efficient laser cooling to occur, the frequency of the laser beam must be tuned to the red sideband. Subsequently, spontaneous emission will occur predominantly at the carrier frequency if the recoil energy of the atom is negligible as compared with the vibrational quantum energy. The overall effect of the mechanism is to cool the ion by one vibrational energy level. To achieve an effective resolved sideband cooling, the process needs to start at sufficiently low (ǹ).
To such an end, the particle is usually first cooled to the Doppler limit. After this, some sideband cooling cycles are applied. In the final act, a measurement is taken or the manipulation of the state is carried out. The narrow quadruple transition which is issued for cooling connects the ground state to a long-lived state, and also the latter has to be pumped out so as to achieve an optimal cooling efficiency.
Raman cooling is a part of atomic physics. It is a sub recoil cooling technique that allows for the cooling of atoms using optical methods that are below the limitations of Doppler cooling. In that sense, Doppler cooling is limited by the recoil energy of a photon given to an atom. This scheme can also be performed in simple optical molasses or in molasses where an optical lattice is superimposed.
Two laser beams can be used to trigger the transition between two hyperfine states of the atom. This is where the first beam excites the atom to a virtual excited state which may be because its frequency is lower than the real transition frequency. The second beam de-excites the atom to the hyperfine state. The transition frequency between the two hyperfine levels is exactly equal to the frequency difference contained by the two beams.
This technique starts off with atoms in a magneto-optical trap. Taking an optical lattice, it is then ramped up in a manner such that an important fraction of the atoms are trapped. Each site can be modeled as a harmonic trap if the lasers of the lattice are powerful enough. The atoms are not in their ground state, so they will be trapped in one of the excited levels of the harmonic oscillator. Putting the atom in the ground state of the harmonic potential at the lattice site is the overall aim of the Raman sideband cooling technique.
We take into consideration a two level atom, the ground state of which has a quantum number of F=1 such that it is a threefold degenerate that has m=-1, 0, or even 1. Using the Zeeman effect, the degeneracy in m is lifted as a magnetic field is added. Therefore, its value is exactly played around with such that the Zeeman splitting between the m=-1 and m=0 and between m=0 and m=1 is equal to the two levels spacing in the harmonic potential which is created by the lattice.
Using the Raman processes, an atom can be transferred to a state where the magnetic moment has decreased by one and the vibrational state has also experienced a decrease by one. After this is experienced, the atoms which are contained in the lowest vibrational state of the lattice potential are pumped optically to the m=1 state.
It may be possible for the atom not to further change its vibrational state during the pumping process since the temperature of the atoms is low enough in relation to the pumping beam frequencies. The atom therefore later ends up in a low vibrational state, and this is how it is cooled down. For the atom to reach this proper efficient transfer to the lower vibrational state at each step, the parameters of the laser which include the power and the timing should thus be properly and carefully tuned in a way that produces the best results.
In a common way, these parameters are different in relation to different vibrational states all because the strength of the coupling depends on the vibrational level. This cooling technique allows one to obtain a high density of atoms all at a low temperature using only optical techniques. A complication may arise from the recoil of photons that drive this transition. However, the complication can be avoided by performing cooling in a Lamb Dicke regime.
This is a method of sub-Doppler laser cooling of atoms. It takes into account the principles from Sisyphus cooling in conjunction with a dark state which has its transition to the excited state not addressed by the resonant lasers. Gray molasses is usually utilized in ultra cold atomic physics experiments on atomic species which have a poorly resolved hyperfine structure, like isotopes of lithium and potassium. This is used to achieve temperatures that are below the Doppler limit.
This Gray molasses can only slow but not trap atoms unlike magneto-optical trap which combines a molasses force with a confining force. Because of this gray molasses is therefore efficient as a cooling mechanism and lasts only milliseconds before further cooling and trapping stages are put into effect. The cooling mechanism of gray molasses depends on two photon and Raman type transitions which are between hyperfine split ground states mediated by an excited state.
Bright and dark states are the orthogonal superposition of the ground states which are constituted since the former couples to the excited state via dipole transitions driven by the laser, and the latter is accessible from the excited state via spontaneous emission. The dark state also evolves into the bright state as neither are Eigenstates of the kinetic energy operator.
It evolves with a frequency that is proportional to the external momentum of the atom. The gradients which are contained in the polarization of the molasses beam create a sinusoidal potential energy landscape for the bright state in which the atoms lose kinetic energy by traveling up to the maxima of the potential energy.
These potential energy maxima coincide with circular polarizations which are capable of executing electric dipole transitions to the excited state. Optical pumping is done to the atoms which are in the excited state and this changes them to the dark state and subsequently evolves them back again to the bright state so as to restart the cycle. The overall effect of the many cycles from bright to excited states to the dark states is to expose the atoms to a Sisyphus-like type of cooling in the bright state and further select the coldest atoms to enter the dark state and escape the cycle.
This technique is a coherent optical nonlinearity that renders a medium transparent within a narrow spectral range around an absorption line. Slow light can be produced as extreme dispersion is created within this transparency window. The propagation of light through an otherwise opaque atomic medium is permitted by a quantum interference effect. A view of electromagnetically induced transparency involves two optical fields of highly coherent light sources such as lasers, and these are tuned to interact with three quantum states of a material.
The probe field is then tuned to a position near resonance at a different transition. The presence of the coupling field will create a spectral window of transparency if the states are properly elected. This created spectral window will be detected by the probe. In this scheme, electromagnetically induced transparency is based on the destructive interference of the transition probability amplitude which is between the atomic states. The cooling process must take into consideration three level atoms with a ground state, an excited state, and a stable state that lies between them.
The excited state is dipole coupled to the stable state and to the ground state. The stable state is driven to the excited state by an intense coupling laser. A weaker cooling laser drives the ground state to the excited state. The electromagnetically induced transparency cooling is realized when the difference in the ground state is equal to the difference in the stable state such that the transition of the carrier lies on the dark resonance of the Fano-like feature.
The red sideband must lie on the narrow maxima of the Fano-like feature, and the blue sideband must lie in the region of low excitation probability. The cooling limit is made lower due to the large ratio of the excitation probabilities. This cooling limit is lowered in relation to Doppler or sideband cooling, as the same cooling rate is assumed for them all.
This is a technique used in laser cooling of atoms as it was proposed to explain the experimental observation involved in the cooling below the Doppler Effect. Doppler cooling allows atoms to be cooled to hundreds of micro Kelvin, whereas polarization gradient allows for atoms to be cooled to a few micro Kelvin or even less. A gradient where the polarization varies in space is created by the superposition of two counter-propagating beams of light that have orthogonal polarizations. This created gradient depends on which type of polarization is used.
In terms where the orthogonal linear polarizations are used, there is a result in the polarization varying between linear and circular polarization in the range which includes half a wavelength. Orthogonal circular polarization can be used and in these, the result is a linear polarization that rotates along the axis of propagation. These configurations both produce the same cooling results but the physical mechanisms involved are different.
In the orthogonal linear configuration, the polarization gradient causes periodic light shifts in the levels of the atomic ground state which allows for the Sisyphus process to occur. In the orthogonal circular propagation, a motion-induced population imbalance in the Zeeman levels is created by the rotating polarization. This is so as the levels are in the ground state and it results in an imbalance of the radiation pressure which opposes the motion of the atom. All these configurations result in sub-Doppler cooling, and they even reach the recoil limit. An atomic gas must be pre-cooled before polarization gradient cooling because the capture range of polarization gradient cooling is lower, although the limit of polarization gradient cooling is lower than that of Doppler cooling.
Polarization gradient cooling is commonly performed using a 3D optical setup. This has three pairs of laser beams that are perpendicular together with an atomic ensemble in the center. Each of these beams is prepared with an orthogonal polarization to its counter-propagating beam. Between the ground state and the excited state of the atom, the laser frequency is detuned from a selected transition.
The cooling process relies on multiple transitions, and because of this extra care must be taken into consideration so that the atom does not fall out of these two states. This extra care can be done by using a second re-pumping laser to pump any atom that falls out back into the ground state of the transition. Before carrying out the polarization gradient cooling, the atoms must be cooled. This is done by using the same setup via Doppler cooling.
The laser intensity must therefore be lowered if the atoms are pre-cooled with Doppler cooling. Also, the detuning must be increased in order to achieve polarization gradient cooling. The time of flight technique can be used to measure the atomic temperature. In this technique, the laser beams are suddenly switched off and the atomic ensemble is allowed to expand. A probe beam is turned on after a set time delay so as to obtain the spatial extent of the ensemble. The rate of expansion is then found by imagining the ensemble at several time delays.
This chapter will discuss the different laser cooling instruments and the types of chillers.
The different laser cooling instruments include:
This is a scientific apparatus that is used in quantum physics to cool a beam of atoms from room temperature or above to a few Kelvin. On its entrance, the average speed of the atoms is about the size of a few hundred m/s. The spread of velocity is also noted and it is in the order of a small amount of m/s. The resulting speed at the exit of the slower is a few 10 m/s which even contains a smaller spread.
The working principle of the Zeeman slower is similar to that of Doppler cooling. An atom that is modeled as a two level atom can be cooled using a laser. If it moves in a specific direction and then encounters a propagating laser beam resonant with its transition, it is very likely to absorb a photon.
The absorption of this photon gives a kick in a direction that is consistent with momentum conservation and brings the atom to its excited state. This contained state is unstable, however, and because of this the atom later decays back to its originally contained ground state through the process of spontaneous emission.
Re-emission of the photon can reoccur, thus increasing the speed of the atom, but its direction will be random. The absorption process decreases the speed in the same direction, so the absorbed photon comes from a source that has a mono direction. The emission process does not result in any change in the speed of the atom, and this is because the emission direction is random.
This is a type of vacuum pump that works by sputtering a metal gutter. Under proper conditions, these ion pumps have an ability of reaching pressures that are as low as 1011 mbar. In its process, it first ionizes gas within the vessel it is attached to and induces a strong electrical potential which may be of a magnitude 3 to 7kV. This potential is used to accelerate the ions into a solid electrode.
Spluttering of these small bits of the electrode is done into the chamber. Due to the combination of chemical reactions, the gasses are trapped. These reactions happen with the surface of the highly reactive sputtered material.
Chillers are found in various categories all based on how the refrigerant releases the heat it absorbs, the function performed by the design, and the compressor type.
The centrifugal chiller makes use of compression so as to convert kinetic energy into static energy. This leads to the pressure and temperature of the refrigerant increasing. The refrigerant is pulled in and compressed using impeller blades.
Laser processes or laser equipment are cooled using laser chillers.
An optimal wavelength has to be maintained for a laser to perform at peak efficiency, and this is achieved by the laser chillers.
Carbon dioxide, ion lasers, and high power exciters have to be accurate and precise. These all depend on a chiller water cooling system.
The refrigerant is changed into vapor by a generator that uses hot water or steam in an absorption chiller.
This vapor then moves to the condenser, where it is sent back to the absorber. The produced vapor is absorbed by a solution which then condenses into a vapor to result in heat.
This type of chiller absorbs heat from water and transfers it into the air. This chiller is used where discharge is not a problem. The heat obtained from circulating chilled water is absorbed in the evaporator. The refrigerant condenses in the condenser as it then releases heat into the air.
This chapter will discuss the applications of laser coolers and laser cooling.
Laser cooling is used in high-resolution spectroscopic measurements such as for frequency standards in optical clocks which are based on ultra cold atoms or ions by the elimination of Doppler broadening.
Laser cooling is also used in ultra-precise measurement of gravitational fields, and these are used in gravitational physics and for the exploration of oil fields. All of this oil field exploration is based on the Doppler shift of free-falling cooled atoms and on Bloch oscillations.
Lithography is another use of laser cooling. This is where accurately controlled structures are produced with cold atomic beams. First, laser cooling is used to create ultra cold atoms for quantum physics experiments performed near absolute zero. In this case, quantum effects such as Bose-Einstein condensation are observed. Some uses may include vibration-free optical refrigeration and radiation-balanced solid-state lasers.
In general, laser cooling refers to the use of dissipative light forces so as to reduce the random motion of contained atoms and also reduce the temperature of small particles like atoms and ions. The temperature attained can result in different magnitudes all depending on the type of mechanism used.
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